The present invention relates to a honeycomb filter and a ceramic filter assembly, and more particularly, to a honeycomb filter formed from a sintered ceramic body and an integral ceramic filter assembly produced by adhering a plurality of honeycomb filters to one another.
The number of automobiles has increased drastically this century. As a result, the amount of gas discharged from automobile engines has continued to increase proportionally. Various substances suspended in the exhaust gas that is emitted, especially from diesel engines, cause pollution and severely affect the environment. Further, recently reported research results have shown that the fine particles suspended in gas emissions (diesel particulates) may cause allergies or decrease sperm counts. Thus, actions to eliminate the fine particles suspended in gas emissions must immediately be taken for the sake of mankind.
Due to this situation, many exhaust gas purification apparatuses have been proposed in the prior art. A typical exhaust gas purification apparatus includes a casing, which is located in an exhaust pipe connected to an exhaust manifold of an engine, and a filter, which is arranged in the casing and has fine pores. In addition to a metal or an alloy, the filter may be formed from ceramic. A cordierite honeycomb filter is a known example of a ceramic filter. Recent filters are often formed from sintered porous silicon carbide body that is advantageous from the viewpoints of heat resistance and mechanical strength, has a high accumulating efficiency, is chemically stable, and has a small pressure loss.
The pressure loss refers to the difference between the pressure value taken upstream of the filter and the pressure value taken downstream of the filter. A main cause of power loss is the resistance the exhaust gas encounters when passing through a filter.
The honeycomb filter includes a plurality of cells extending along the axial direction of the honeycomb filter. When the exhaust gas passes through the filter, the walls of the cells trap fine particles. This removes fine particles from the exhaust gas.
However, the honeycomb filter, which is made of a sintered porous silicon carbide body, is vulnerable to thermal impacts. Thus, larger filters are liable to crack. Accordingly, a technique for manufacturing a large ceramic filter assembly by integrating a plurality of small filters has recently been proposed to prevent breakage resulting from cracks.
A typical method for manufacturing a ceramic filter assembly will now be discussed. First, ceramic raw material is continuously extruded from a mold of an extruder to form an elongated square honeycomb molded product. After the honeycomb filter is cut into pieces of equal length, the cut pieces are sintered to form a filter. Subsequent to the sintering process, a plurality of the filters are bundled and integrated by adhering the outer surfaces of the filters to each other with a ceramic seal layer having a thickness of 4 to 5 mm. This completes the desired ceramic filter assembly.
A mat-like thermal insulative material, made of ceramic fiber or the like, is wrapped about the outer surface of the ceramic filter assembly. In this state, the assembly is arranged in a casing, which is located in an exhaust pipe.
However, in the prior art, there is a shortcoming in that the fine particles trapped in the ceramic filter assembly do not burn completely and some of the fine particles remain unburned. Accordingly, the efficiency for processing the exhaust gas is low.
Further, the honeycomb filter of the prior art has corners. Thus, there is a tendency of stress concentrating on the corners of the outer surface and chipping the corners. Further, the seal layer may crack and break the ceramic filter assembly from the corners thereby damaging the entire ceramic filter assembly. Even if the assembly does not break, there is a shortcoming in that leakage of the exhaust gas may decrease the processing efficiency.
During usage of the filter assembly, a high temperature difference between the honeycomb filters may cause thermal stress to crack the honeycomb filters and break the entire assembly. Thus, the strength of each honeycomb filter must be increased to increase the strength of the honeycomb filter assembly.
The prior art ceramic filter assembly as a whole has a rectangular cross-section. Thus, the periphery of the assembly is cut so that the assembly as a whole has a generally round or oval cross-section.
However, the filter has a plurality of cells. Thus, if the periphery of the assembly is cut, the cell walls are exposed from the peripheral surface subsequent to the cutting. This forms lands and pits on the peripheral surface. Thus, even if the assembly is accommodated in the casing with the thermal insulative material attached to the peripheral surface of the assembly, gaps are formed in the longitudinal direction of the filters. Thus, exhaust gas tends to leak through the gaps. This lowers the processing efficiency of the exhaust gas.
With regard to diesel particulates trapped in the honeycomb filter, it has been confirmed that particulates having a small diameter have a high lung attaching rate and increase the risk to health. Thus, there is great need to trap small particulates.
However, when the pore diameter and the porosity of the honeycomb filter are small, the honeycomb filter becomes too dense and hinders smooth passage of the exhaust gas, which, in turn, increases the pressure loss. This lowers the driving performance of the vehicle, lowers fuel efficiency, and deteriorates the driving performance.
On the other hand, if the pore diameter and porosity rate are increased, the above problems are solved. However, the number of openings in the honeycomb filter becomes too large. Thus, fine particles cannot be trapped. This decreases the trapping efficiency. Further, the mechanical strength of the honeycomb filter becomes low.
It is a first object to provide a ceramic filter assembly having an improved exhaust gas processing efficiency.
It is a second object of the present invention to provide a ceramic filter assembly having superior strength.
It is a third object of the present invention to provide a ceramic filter assembly that prevents fluid leakage from the peripheral surface.
It is a fourth object of the present invention to provide a honeycomb filter having small pressure loss and superior mechanical strength.
A first perspective of the present invention is an integral ceramic filter assembly produced by adhering with a ceramic seal layer outer surfaces of a plurality of filters, each of which is formed from a sintered porous ceramic body. The seal layer has a thickness of 0.3 mm to 3 mm and a thermal conductance of 0.1W/mK to 10W/mk.
A second perspective of the present invention is an integral ceramic filter assembly produced by adhering with a ceramic seal layer outer surfaces of a plurality of elongated polygonal honeycomb filters, each of which is formed from a sintered porous ceramic body. Round surfaces are defined on chamfered corners of the outer surface of each honeycomb filter, and the round surfaces have a radius of curvature R of 0.3 mm to 2.5 mm.
A third perspective of the present invention is an integral ceramic filter assembly produced by adhering with a ceramic seal layer outer surfaces of a plurality of filters, each of which is formed from a sintered porous ceramic body. The ceramic filter assembly includes a ceramic smoothing layer applied to the outer surface of the assembly, which as a whole has a generally circular cross-section or generally oval cross-section.
A fourth perspective of the present invention is an integral ceramic filter assembly produced by adhering with a ceramic seal layer outer surfaces of a plurality of elongated honeycomb filters, each of which is formed from a sintered porous ceramic body. A ratio L/S between a filter length L in a flow direction of a processed fluid and a filter cross-section S in a direction perpendicular to the flow direction is 0.06 mm/mm2 to 0.75 mm/mm2.
A fifth perspective of the present invention is an integral honeycomb filter assembly produced by adhering with a ceramic seal layer outer surfaces of a plurality of honeycomb filters, each of which has a plurality of cells defined by a cell wall and which purifies fluid including particulates with the cell wall. A specific surface area of grains forming the cell wall is 0.1 m2/g or more.
A sixth perspective of the present invention is an elongated honeycomb filter formed from a sintered porous ceramic body. A ratio L/S between a filter length L in a flow direction of a processed fluid and a filter cross-section S in a direction perpendicular to the flow direction is 0.06 mm/mm2 to 0.75 mm/mm2.
A seventh perspective of the present invention is a honeycomb filter formed from a sintered porous ceramic body. An average pore diameter of the honeycomb filter is 5 to 15 xcexcm, an average porosity is 30 to 50%, and the honeycomb filter has 20% or more of through pores.
An eighth perspective of the present invention is a honeycomb filter having a plurality of cells defined by a cell wall and purifying fluid including particulates with the cell wall. A specific surface area of grains forming the cell wall is 0.1 m2/g or more.
FIG. 1 is a schematic view showing an exhaust gas purification apparatus according to a first embodiment of the present invention.
FIG. 2 is a perspective view showing a ceramic filter assembly of the exhaust gas purification apparatus of FIG. 1.
FIG. 3 is a perspective view showing a honeycomb filter of the ceramic filter assembly of FIG. 2.
FIG. 4 is an enlarged cross-sectional view showing the main portion of the exhaust gas purification apparatus of FIG. 1.
FIG. 5 is an enlarged cross-sectional view showing the main portion of the ceramic filter assembly of FIG. 2.
FIG. 6 is an enlarged cross-sectional view showing the main portion of a ceramic filter assembly of a first modified example.
FIG. 7 is a perspective view showing a honeycomb filter according to a second embodiment of the present invention.
FIG. 8 is an enlarged cross-sectional view showing the main portion of a ceramic filter assembly.
FIG. 9 is an enlarged cross-sectional view showing the main portion of a ceramic filter assembly according to a first modified example.
FIG. 10 is a perspective view showing the honeycomb filter according to the first modified example.
FIG. 11 is a perspective view showing a honeycomb filter according to a second modified example.
FIG. 12 is a perspective view showing a honeycomb filter according to a third modified example.
FIG. 13 is a side view showing a ceramic filter assembly according to a third embodiment of the present invention.
FIGS. 14(a) to 14(c) are schematic perspective views illustrating a manufacturing process of the ceramic filter assembly of FIG. 13.
FIG. 15 is a side view showing a ceramic filter assembly according to a modified example.
FIG. 16 is a perspective view of a ceramic filter assembly according to a fourth embodiment of the present invention.
FIG. 17 is a perspective view showing a filter of the ceramic filter assembly 3 of FIG. 16.
FIG. 18(a) is a schematic cross-sectional view showing the filter of FIG. 17, and
FIG. 18(b) is a schematic side view showing the filter of FIG. 17.
FIG. 19 is a perspective view showing a honeycomb filter provided with a honeycomb structure according to fifth and sixth embodiments of the present invention.
FIG. 20 is a cross-sectional view showing the filter 59 of FIG. 19 taken along line 20xe2x80x9420.
FIG. 21 is an enlarged cross-sectional view showing the main portion of an exhaust gas purification apparatus.
FIG. 22 is a perspective view showing a ceramic filter assembly.